That's a rather subjective question, as people can differ on their own judgments about the matter. Suffice it to say that William himself thought he was justified. He had known the previous king, Edward the Confessor, for many years. He had met and admired the man many years before as a child, when the adult Edward and his brother stayed in Normandy, exiled from Britain. According to William, he visited Edward in England (about 1052 or 1053), and Edward at that time told him he wanted William to be his successor. Then, around 1062 or 1063, Harold (who received the throne upon Edward's death a few years later) visited Normandy -- possibly by accident, the winds blew his ship across. While in William's company (somewhat a "hostage" Harold felt), William forced Harold to swear -- on sacred Christian relics -- to give William the throne and assist William; also, for Harold to marry William's young daughter, who was then only about 10 years old. When in early 1066 Harold blatantly ignored the sacred oaths he had made to William -- taking the kingship for himself, and marrying another, William was outraged -- and thus felt himself justified in invading England. He apparently even made further written communication with Harold, offering Harold an additional chance to back out, before William came over with his army. The rest, as they say, is history. The history tells me that there was some cause for William, as I can understand both his viewpoint and that of the English and their leader Harold. Edward desired that Harold would be best for England's future, but clearly God had a different purpose, in allowing it so that the Normans conquered England.

Ultimately from the outside looking in I would say no since he did it basically to win the throne and caused the feudal system to reign supreme in Europe. I'm not saying his conquest turned out all bad for England, just trying to put it modern terms something that happened a long time ago. We can say most of what they did so many centuries ago were cruel, wrong, barbaric, etc., so I am not putting too much blame on William I for everything bad that happened after his reign.